In a wireless environment, devices must share the RF spectrum while operating at an acceptable level of performance. As the number of wireless applications continues to grow, so does the numbers of devices, networks and systems contending for the communications spectrum, making the management of the RF spectrum an important issue.
Use of the RF spectrum is regulated by governmental entities in most countries. Currently, there are dedicated or licensed portions as well as unlicensed portions of the communications spectrum. Certain bands of the spectrum may be overloaded, such as cellular network bands, while other bands may be underused, such as television and radio broadcast bands. Because the unlicensed bands of the spectrum may be accessed freely, these bands tend to be heavily used. In contrast, only a small portion of the licensed bands is being used. Moreover, a specific television (TV) market may have only a few TV stations, leaving the rest of the licensed spectrum unused, or a radio or TV station may cease broadcasting at certain times of day.
The underuse of the licensed spectrum, together with the evolution of TV broadcasting from analog to digital, has lead regulatory bodies in some countries to open up some bands of the RF spectrum to secondary (unlicensed) users, provided such secondary users avoid interference with primary (licensed) users. As each TV station operating in a given market uses only a limited number of channels within the dedicated TV band, some digital channels remain unused in the respective area: this locally available spectrum is called “white space”. Devices that use white space for wireless communication are often referred to as cognitive radios (CRs), secondary user systems, or white space devices. White space devices are required to identify free, or unused, channels in the licensed spectrum, and/or identify channels that are occupied by a primary user. If a primary user service is identified, the secondary user is required to seek other channel(s) or delay its use of the channel(s) until the primary user ceases to communicate.
In the United States, Federal Communications Commission (FCC) order FCC 08-260 specifies that devices operating in the TV band must not interfere with incumbent services such as TV broadcasts and wireless microphones. This order requires that signals radiated by any secondary devices or equipment operating in the Advanced Television Systems Committee (ATSC) spectrum must follow the FCC regulations so that the quality of the primary TV service will not be degraded by the signals present in nearby channels. Therefore white space devices need to be equipped with spectrum usage and management capabilities in order to meet the FCC's requirements.
To mitigate the interference between the increasing number of wireless devices vying for use of the spectrum, a variety of detection and avoidance schemes have been developed. However, the solutions currently proposed for spectrum management by cognitive radio systems result in implementation complexity by increasing the complexity of the systems, and in spectrum waste due to the cognitive radios using less than the full bandwidth available in the RF band.
Some techniques for detecting activity of wireless devices in a portion of interest of the radio spectrum include the use of quiet periods provided specifically for sensing the spectrum. During a quiet period, one or more wireless devices sharing a channel or a cell, refrain from transmitting and use the quiet period to detect other wireless devices, if any, that which they might interfere. This technique is proposed in the IEEE 802.22 draft standard for Wireless Regional Area Networks (WRANs). However, use of quiet period synchronization results in resource wastage and reduced quality of service (QoS), and current proposals do not permit the quiet period to be shortened when further detection or sensing is unnecessary.
According to another technique for detecting activity in the RF spectrum, the devices operating in a cell may employ channel-hopping or Dynamic Frequency Hopping (DFH): using a subset of the available channels in a band and, hopping to a different subset at regular intervals, after checking that the new subset is not already in use. In this way, the secondary users avoid hopping onto channels already in use by a primary user and, within a predetermined period, hop off of channels that a primary user has begun using. Such hopping techniques may also be used among sub-carriers within a single channel.
Another option for reallocation of the communications band involves the use of wireless networks adapted to dynamically access the communications spectrum. For example, dynamic spectrum access (DSA) wireless networks may be implemented in the licensed portions of the communications spectrum.
There is therefore a need for a more efficient technique of sensing the spectrum occupancy and managing the free spectrum in both time and frequency.